KR101356862B1 - Optical transimpedance amplifier for optical receiver in a optical communication system - Google Patents

Abstract

The present invention relates to a photodiode (PD) for receiving an optical signal and generating a current signal by photoelectric conversion in an optical communication system, and converting a current signal input from the photodiode into a voltage signal. Trans-Ampedance Amplifier (TIA), automatic gain adjuster for adjusting the feedback resistance of the preamplifier, photodiode parallel capacitor reduction unit for reducing the current of the parallel capacitor of the photodiode, and gain increase and decrease of the preamplifier It includes a bandwidth adjustment unit for reducing the bandwidth increase and decrease according to.

Optical communications, light receiving elements, preamplifiers, comparators, capacitors, bandwidth

Description

OPTICAL TRANSIMPEDANCE AMPLIFIER FOR OPTICAL RECEIVER IN A OPTICAL COMMUNICATION SYSTEM}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical communication systems, and more particularly, to a design of a pre-amp (TransImpedance Amplifier) circuit for amplifying the output current of a light receiving element into a voltage in an optical communication optical receiver.

In general, a communication system that transmits a signal to a light emitting device such as a laser diode and receives a signal to a light receiving device such as a photo diode is called an optical communication system. Such an optical communication system is a technology suitable for transmitting a large amount of data such as a broadcast signal, and a representative example of the optical communication system may be a passive optical network (PON). Passive optical subscriber network has a tree-structured distributed topology through an optical splitter between one central station transceiver (OLT) and multiple subscriber optical transceivers (ONT). This is a subscriber network.

In the passive optical subscriber network, the central station transceiver (OLT) converts, for example, an analog and / or digital broadcast signal into an optical signal having a predetermined wavelength, and multiplexes it to an optical distribution unit, and the optical distribution unit transmits the optical station. The optical signal transmitted from the transceiver (OLT) is distributed to the plurality of subscriber transceivers (ONT) and transmitted. The plurality of subscriber transceivers ONT optically converts the received optical signals into analog and / or digital broadcast signals and transmits them to a set top box or a computer device (not shown) of each subscriber.

Here, the optical receiver (not shown) in the subscriber transceiver ONT, that is, the photoelectric converter, generally adjusts the intensity of the optical signal to stably adjust the output level during photoelectric conversion of the optical signal received from the central station transceiver OLT. And a gain adjusting circuit for adjusting the gain of the photoelectrically converted output signal according to the detected intensity of the optical signal.

1 is a circuit diagram of a gain control device provided in an optical receiver of a general optical communication system. For example, the signal intensity of a received optical signal is measured to gradually adjust the resistance values of the feedback resistors R1, R2, and R3. A variable gain amplifier circuit is shown. The device of FIG. 1 is disclosed in US Pat. The configuration is briefly described as follows.

That is, the circuit of FIG. 1 includes a preamplifier 101 for converting a received optical signal into a voltage signal, and between the input terminal and the output terminal 103 of the preamplifier 101 to gradually adjust the gain of the preamplifier 101. A plurality of feedback resistors R1, R2, and R3 connected in parallel to the plurality of feedback resistors R1, R2, and R3 and the output terminals of the preamplifier 101, respectively, and are connected to a predetermined control signal. And a plurality of buffer amplifiers 105, 107, and 109 which are turned on / off according to the present invention.

As shown in FIG. 1, when feedback resistors R1, R2, and R3 that adjust the gain of the preamplifier 101 are connected in parallel, a control signal is selectively applied to each of the buffer amplifiers 105, 107, and 109. The amplifiers 105, 107 and 109 are selectively switched on / off to change the summed resistance values of the feedback resistors R1, R2 and R3. As a result, the output gain of the preamplifier 101 is adjusted according to the resistance values of the feedback resistors R1, R2, and R3. For example, when a control signal is applied such that only two buffer amplifiers 105 and 109 are conducting, and the other buffer amplifier 107 is turned off, the line passing through R2 and the buffer amplifier 107 has infinite resistance. Therefore, the gain of the preamplifier 101 is determined by the resistance value of R1 and R3 connected in parallel.

However, in the case of the gain control device of FIG. 1, the gain is adjusted by using the summed resistance values of the plurality of feedback resistors, so that gain adjustment is discrete. That is, the stage of gain adjustment is determined in proportion to the number of feedback resistors, but if the number of feedback resistors is increased to subdivide the stage of gain adjustment, the number of buffer amplifiers also increases proportionally and is used to generate a control signal. Since additional circuits (not shown) also increase in proportion, there is a large constraint on the continuous (linear) gain control depending on the intensity of the optical signal.

In addition, in order to generate a control signal applied to the buffer amplifier, the gain adjusting apparatus of FIG. 1 configures a separate circuit for detecting and processing the magnitude of the input optical signal from the output voltage of the preamplifier. It should be controlled on / off. In this case, however, the circuit becomes complicated and high accuracy is required. Thus, in order to subdivide the gain adjustment step, there is a difficulty in that a circuit for generating a control signal in proportion to the number of gain adjustment steps must be additionally provided.

Meanwhile, the gain of the preamplifier is determined by the value of the feedback resistor, and the bandwidth of the preamplifier changes accordingly. Therefore, in the light of the principle that the product of the bandwidth and the gain is constant, the larger the total value of the feedback resistor, the larger the gain, and the smaller the total value of the feedback resistor, the smaller the gain, the larger the bandwidth.

Accordingly, there is a need for a preamplifier design that can solve the above problems and reduce the bandwidth increase / decrease ratio due to gain increase and decrease to perform stable operation.

The present invention reduces the combination of the feedback resistor of the preamplifier, which has been a problem in the prior art, and reduces the bandwidth increase / decrease ratio according to the gain increase and decrease, thereby reducing the parallel capacitor of the preamplifier circuit and the photodiode capable of performing stable operation. To provide a way to.

In order to achieve this, the present invention provides a photodiode (PD) for receiving an optical signal and generating a current signal by photoelectric conversion in a preamplifier circuit for an optical receiver in an optical communication system, and the current input from the photodiode. A preamplifier (TIA) for converting a signal into a voltage signal, an automatic gain adjuster for adjusting a feedback resistance of the preamplifier, and a photodiode parallel capacitor for reducing current of the parallel capacitor of the photodiode. And a bandwidth adjusting unit for reducing a bandwidth increase / decrease range according to gain increase or decrease of the preamplifier.

The automatic gain control unit includes a bottom hold circuit connected to an output terminal of the preamplifier for detecting a low level of an output signal of the preamplifier, and two input terminals, and having two input terminals. A comparator for receiving an output signal at an input terminal and receiving a reference voltage (Vref) at a + input terminal and comparing the two values; a fourth resistor and a fifth resistor connected in parallel to the preamplifier; And a sixth transistor connected in series to perform a switching operation of the fourth resistor according to the output signal of the comparator.

The photodiode parallel capacitor reduction unit includes a first capacitor connected in series to an anode end of the photodiode, a third resistor connected in series to the first capacitor, a collector end connected to a power supply, and a base end connected to the first capacitor. And a fourth transistor connected between a third resistor and an emitter terminal connected to a cathode terminal of the photodiode.

The bandwidth adjusting unit may include a first transistor switched according to an output signal of the comparator, and a second transistor, a third transistor, and a fifth transistor constituting a current mirror circuit.

The preamplifier according to the embodiment of the present invention reduces the parallel capacitor (PD) of the photodiode (PD) to increase the bandwidth, and can realize a bandwidth change insensitive to the increase or decrease of the feedback resistance of the automatic gain control device. It works.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an apparatus and an operation method of the present invention will be described in detail with reference to the accompanying drawings.

2 is a circuit diagram illustrating a preamplifier for an optical receiver according to an embodiment of the present invention. The preamplifier circuit for an optical receiver according to an embodiment of the present invention includes a photo diode (PD) for generating a current signal by photoelectric conversion of an input optical signal, and a voltage signal for a current signal input from the photo diode. A transimpedance amplifier (TIA) 211 for converting the signal into a power amplifier; an automatic gain control unit for adjusting a feedback resistance of the preamplifier 211; and a photodiode for reducing current of a parallel capacitor of the photodiode. And a parallel capacitor reducing unit and a bandwidth adjusting unit for reducing a bandwidth increase / decrease range according to gain increase or decrease of the preamplifier.

Referring to FIG. 2, the photodiode is represented by an equivalent circuit, a current source ipd generating a current output from the photodiode, and a second capacitor C2 connected in parallel to the current source ipd. It can be represented as.

The automatic gain control unit is connected to an output terminal of the preamplifier 211 and a bottom hold circuit (BH: Bottom Hold) 212 for detecting a low level of the output signal of the preamplifier 211, two A comparator 213 having two input terminals and receiving an output signal of the bottom hold circuit 212 at an input terminal and a reference voltage Vref at a + input terminal and comparing the two values; The fourth resistor R4 and the fifth resistor R5 connected in parallel to the preamplifier 211 and the fourth resistor R1 are connected in series to each other in accordance with the output signal of the comparator 213. The sixth transistor TR6 performs a switching operation.

The parallel capacitance reduction unit of the photodiode has a first capacitor C1 connected in series to the anode end of the photodiode, a third resistor R3 connected in series with the first capacitor, and a collector end connected to a power source and the base A stage includes a fourth transistor TR4 connected between the first capacitor and the third resistor and an emitter terminal connected to the cathode of the photodiode.

The preamplifier 211 bandwidth adjusting unit includes a first transistor TR1 switched according to an output signal of the comparator 213, a second transistor TR2 and a third transistor constituting a current mirror circuit. TR3) and a fifth transistor TR5.

The operation of the preamplifier circuit will be described with reference to the configuration of the preamplifier circuit described above.

The gain of the preamplifier 211 is determined by the value of its feedback resistance. In other words, when a fixed feedback resistor is used, when the input optical signal is large (Overload), it is generally outside the input possible range of the limiting amplifier (LA), which is the next stage after the preamplifier 211, so that signal distortion may occur at the final output. Can be. Therefore, when a large optical signal is input, it is necessary to reduce the gain by reducing the feedback resistance value of the preamplifier 211.

In this manner, in the present invention, in the case of the preamplifier 211 having a negative gain in general, the output of the preamplifier 211 is lower than that of the direct current (DC) state when a signal is input. A bottom hold circuit 212 is used at the output of 211 to detect an envelope of the signal. That is, as the magnitude of the input optical signal increases, the voltage value detected by the bottom hold circuit 212 is lowered. Therefore, when the magnitude of the detected signal is smaller than Vref, which is an arbitrary comparison voltage value, the comparator outputs Logic 'High' to 'ON' the sixth transistor TR6 used as a switch. As a result, the fourth resistor and the fifth resistor are connected in parallel, and the total feedback resistor becomes a resistance value obtained by connecting two resistors in parallel. On the contrary, when the size of the input optical signal is small, the sixth transistor TR6 is 'OFF' in the same principle, so that the entire feedback resistance becomes the fifth resistor.

In summary, when the small optical signal is input, the gain is the fifth resistor, and when the large optical signal is input, the gain is the resistance value obtained by connecting the fourth resistor and the fifth resistor in parallel. will form a loop.

Operation of the reduction unit of the second capacitor C2, which is a parallel capacitor of the photodiode, is as follows. The current ipd generated by the optical signal input to the photodiode is represented by ic2 + ic1 + itia, of which the current is increased when the impedance of the third resistor R3 and the first capacitor C1 connected in series with ic1 is increased. ic1 becomes negligible. Thus, ipd = ic2 + itia.

In order to obtain a large gain and bandwidth, we need to reduce ic2 so that most of the generated ipd becomes itia and flows to the feedback resistor. As is well known, the amount of current flowing into the capacitor (Δq) is proportional to the size (C) of the capacitor and the voltage change (Δv) at both ends. That is, Δq = C * Δv. Since the C value is fixed, in order to reduce the amount of current flowing in, it is necessary to reduce the voltage change (Δv) at both ends. That is, in order to reduce ic2, the voltage difference between Va and Vc should be reduced, and the relationship between Va and Vc is shown in Equation 1 below. In other words, the higher the frequency used, the smaller the voltage difference between Va and Vb. However, Vc is smaller than Vb by the ratio of gm / (gm + gmb) because transistor TR4 is a source follower. Therefore, a slightly smaller signal of the same phase than Va is applied to Vc, thereby significantly reducing the value of ic2. Therefore, as a result, the function of reducing the value of the second capacitor (C2).

In Equation 1, C1 is a capacitor value of the first capacitor, s is a frequency, R3 is a resistance value of a third resistor, gm is a transconductance, and gmb is a body transconductance.

4 is a circuit diagram illustrating an equivalent circuit of a preamplifier circuit for an optical receiver according to an exemplary embodiment of the present invention. The preamplifier bandwidth adjusting unit will be described with reference to the equivalent circuit of FIG. 4 and Equations 2, 3, and 4 below.

In Equation 2, A is-(Vout / Vin), s is complex frequency (s = jw), w = 2πf, Rf is the total feedback resistance value of the preamplifier, and Ro is shown in FIG. The resistance value of the resistor Ro in the equivalent circuit.

Referring to Equation 2, the current ipd generated in the photodiode is equal to the sum of itia, ic2, and ic1. However, as described above, since ic1 is negligible, ipd = itia + ic2. After arranging this as a function of Vout as shown in Equation 2, the input / output transfer function is calculated as shown in Equation 2 using the gain of the preamplifier as Vout / ipd.

Therefore, if the denominator in the denominator (pole frequency), that is, the frequency to find the drop to 3dB can be found as shown in Equation 3 below.

As can be seen from Equation 3, the 3dB frequency depends on Rf and Ro. Conventional conventional preamplifiers adjust the Rf to increase or decrease the gain, the 3dB frequency changes inversely, but in the present invention can have one more variable called Ro according to the characteristics of the present invention to compensate for this. do.

In Equation 4, k 'is u * Cox, u is mobility, Cox is a capacitance of a gate oxide film, W is a channel width of transistor TR4, and L is a channel length of transistor TR4. .

As shown in Equation 4, Ro is expressed by the inverse of gm. In order to change gm, it is effective to change i4 which is a bias current of TR4.

That is, in one embodiment of the present invention, when a small optical signal is input, Rf is increased because the gain is increased by the automatic gain adjusting unit. Therefore, it is necessary to reduce Ro in order to prevent the 3dB frequency from being excessively reduced. To reduce Ro, we need to grow gm so i4 can be increased.

When the small optical signal is input as described above, since the output of the comparator 213 is 'Logic Low', the third transistor TR3 and the fifth transistor TR5 are turned off by turning off the first transistor TR1. It will all work. Therefore, when it is assumed that the sizes of the two transistors are the same as that of the second transistor TR2, the current twice as much as Is is determined by the current mirror circuit as an i4 value which is a bias current of the fourth transistor TR4.

On the contrary, when a large optical signal is input, it is necessary to raise Ro by the same principle. To increase Ro, you need to reduce gm, so reduce i4. In order to reduce i4, since a large optical signal input is input in this case, since the output of the comparator 213 is 'Logic High', the first transistor TR1 is 'ON' so that the fifth transistor TR5 has a 'Logic' gate. Low 'is applied, so the fifth transistor TR5 is' OFF' and does not operate. Therefore, the Is value is determined by the current mirror circuit as an i4 value which is a bias current of the fourth transistor TR4.

In addition to the above-described method, a method of changing the channel width W and the channel length L of the transistor to change Ro may be used.

As described above, the bandwidth increase / decrease range according to the gain change can be reduced by the operation of the bandwidth adjusting unit according to the aspect of the present invention, thereby enabling the design of the preamplifier having a small bandwidth change according to the strength of the input signal.

3 is a circuit diagram illustrating a preamplifier for an optical receiver according to another embodiment of the present invention.

The preamplifier circuit for an optical receiver according to another embodiment of the present invention includes a photodiode for generating a current signal by photoelectric conversion of an input optical signal, and a preamplifier for converting a current signal input from the photodiode into a voltage signal. 311, an automatic gain adjusting unit for adjusting the feedback resistance of the preamplifier 311, a parallel capacitor reducing unit of the photodiode, and a preamplifier 311 bandwidth adjusting unit.

In the second embodiment of the present invention illustrated in FIG. 3, since the automatic gain control unit and the parallel capacitor reduction unit of the photodiode are the same as the configuration and operation of the first embodiment of the present invention described with reference to FIG. 2, a detailed description thereof is omitted. Let's do it.

According to another exemplary embodiment of the present invention, the preamplifier bandwidth adjusting unit includes a first resistor R1 connected in series with a second transistor TR2, a third transistor TR3, and a current Is flowing in a current mirror circuit. ), A second resistor R2, an inverter 331 connected in series to the output terminal of the comparator 313, and an output signal of the comparator 313 connected in parallel with the second resistor and passing through the inverter 331. And a first transistor for performing a switching operation.

Referring to the bandwidth adjustment operation of the preamplifier for the optical receiver according to another embodiment of the present invention, another embodiment of the present invention is connected to the first resistor and the second resistor in series without using a fixed resistor in generating Is. After that, the first transistor connected to the inverter 331 is connected in parallel to the second resistor, and when the large optical signal is input, the first transistor is 'OFF', and the first resistor and the second resistor are connected, so that Is is small. When the small optical signal is input, the first transistor is 'ON', and the Is becomes large because the current passes through the first resistor and passes through the first transistor. Is thus determined becomes the bias current of the fourth transistor by the current mirror circuit. Therefore, according to the characteristics of the present invention, it is possible to reduce the bandwidth increase / decrease range according to the gain change, thereby enabling a preamplifier design having a small bandwidth change according to the strength of the input signal.

As described above, the configuration and operation of a preamplifier circuit for an optical receiver in an optical communication system according to an embodiment of the present invention can be made. Meanwhile, the above description of the present invention has been described with reference to specific embodiments. It can be carried out without departing from the scope.

1 is a circuit diagram of a gain control device provided in an optical receiver of a general optical communication system.

2 is a circuit diagram illustrating a preamplifier for an optical receiver according to an embodiment of the present invention.

3 is a circuit diagram illustrating a preamplifier for an optical receiver according to another embodiment of the present invention.

4 is a circuit diagram illustrating an equivalent circuit of a preamplifier circuit for an optical receiver according to an exemplary embodiment of the present invention.

Claims (15)

In the preamplifier circuit for an optical receiver in an optical communication system, A photo diode (PD) including a current source that receives an optical signal and performs photoelectric conversion and generates a current, and a second capacitor connected in parallel to the current source; A transamplifier amplifier (TIA) for converting the current input from the photodiode into a voltage; An automatic gain adjuster for adjusting a feedback resistance of the preamplifier; A photodiode parallel capacitor reduction unit including a fourth transistor and reducing a current input by the second capacitor using the fourth transistor according to a decrease in the voltage difference across the photodiode; A bandwidth including a first transistor and a range of bandwidth increase and decrease according to gain increase and decrease of the preamplifier using the first transistor according to on or off of the first transistor determined by an output signal of a comparator. A preamplifier circuit for an optical receiver in an optical communication system, characterized in that it comprises a bandwidth adjustment unit for reducing the frequency. The method of claim 1, wherein the automatic gain control unit, A bottom hold circuit connected to an output terminal of the preamplifier for detecting a low level of an output signal of the preamplifier; A comparator having two input terminals and receiving an output signal of the bottom hold circuit at an input terminal and a reference voltage Vref at a + input terminal and comparing the two values; Fourth and fifth resistors connected in parallel to the input terminal and the output terminal of the preamplifier, respectively; An emitter stage and a cathode stage are connected in series with the fourth resistor, a base stage is connected with an output terminal of the comparator, is connected in parallel with the fifth resistor, and performs an on or off switching operation according to the output signal of the comparator. And a sixth transistor comprising: a preamplifier circuit for an optical receiver in an optical communication system. 3. The preamplifier circuit according to claim 2, wherein the reference voltage is a preset comparison voltage value capable of determining an overload of an input optical signal. 3. The transistor of claim 2, wherein the sixth transistor is turned on when the output signal of the comparator is 'Logic High'. And the sixth transistor is turned off if the output signal of the comparator is 'Logic Low'. The method of claim 1, wherein the photodiode parallel capacitor reduction unit, A first capacitor connected in series with an anode end of the photodiode, A third resistor connected in series to the cathode terminals of the first capacitor and the fourth transistor; And a fourth transistor having a collector end connected to a power source, a base end connected between the first capacitor and a third resistor, and an emitter end connected to the cathode end of the photodiode. Preamplifier circuit. 6. The method of claim 5, wherein the current flowing through the second capacitor is proportional to the voltage difference across the photodiode, and the voltage difference across the photodiode decreases as a higher frequency is used to reduce the current flowing through the parallel capacitor of the photodiode. Further reducing the preamplifier circuit for an optical receiver in an optical communication system. The method of claim 1, wherein the bandwidth adjustment unit, A second transistor, a third transistor, and a fifth transistor constituting a current mirror circuit; The base end of the third transistor is connected to the base end of the second transistor and the cathode end of the second transistor, and the cathode end of the third transistor is the cathode end of the fifth transistor, the cathode end of the photodiode, and the fourth transistor. The emitter terminal of the third transistor is connected to the emitter terminal of the transistor, and the emitter terminal of the first transistor, the second transistor, and the fifth transistor is connected to each emitter terminal, and the base terminal of the fourth transistor is connected to the photodiode. A first capacitor connected in series to an anode terminal, and a cathode terminal of the fourth transistor is connected to a power supply VDD and a third resistor connected in series to the first capacitor. . 8. The preamplifier circuit for optical receivers according to claim 7, wherein the input / output transfer function of the preamplifier circuit is as shown in Equation 5 below.

In Equation 5, i _pd is a current value generated by the current source, V _out is an output voltage value of the preamplifier, V _in is a voltage value according to the current generated in the photodiode, and A is-(V _out / V _in ), s is complex frequency (s = jw), w = 2πf, C ₂ is the capacitor value of the second capacitor, R _f is the feedback resistance value of the preamplifier, R _o Is an equivalent resistance value of the fourth transistor.  10. The preamplifier circuit of claim 8, wherein a frequency falling to 3 dB, which is a pole frequency in Equation 5, is represented by Equation 6 below.

10. The preamplifier circuit for optical receivers according to claim 9, wherein R _o is expressed by Equation 7 below.

In Equation 7, k 'is u * Cox, u is carrier mobility, Cox is capacitance of the gate oxide of the fourth transistor, g _m is transconductance, and g _mb Is body transconductance, W is the channel width of the fourth transistor, L is the channel length of the fourth transistor, and I ₄ is the bias current of the fourth transistor. The preamplifier circuit of claim 10, wherein the preamplifier circuit changes R _o to reduce a bandwidth increase or decrease according to a change in gain, and I ₄ , which is a bias current of the fourth transistor through the current mirror circuit to change R _o . A preamplifier circuit for an optical receiver in an optical communication system, characterized in that for varying. The method of claim 11, wherein the bandwidth adjustment unit, When the output of the comparator is 'Logic Low', adjust the value of I ₄ to be twice the value of Is, the input current value of the second transistor, When the output of the comparator is 'Logic High', the pre-amplifier circuit for an optical receiver in the optical communication system, characterized in that the I ₄ value is adjusted to be equal to the Is value. delete The method of claim 1, wherein the bandwidth adjustment unit, A second transistor and a third transistor constituting the current mirror circuit; A first resistor and a second resistor connected in series with each other, in which an Is, an input current value of the second transistor, flows in the current mirror circuit; An inverter connected in series with the output terminal of the comparator; And a first transistor connected in parallel to the second resistor and performing an on or off switching operation according to an output signal of the comparator through the inverter, The base end and the cathode end of the second transistor are connected to the base end of the third transistor, the emitter end of the second transistor is connected to the emitter end of the third transistor, and the cathode end of the third transistor is connected to the phototransistor. A preamplifier circuit for an optical receiver in an optical communication system, characterized in that connected to the cathode end of the diode. 15. The method of claim 14, wherein the bandwidth adjustment unit, When the 'Logic High' signal is output from the comparator, the first transistor is 'OFF' so that the first resistor and the second resistor are connected, and the Is becomes smaller than when the 'Logic Low' signal is output. When the 'Logic Low' signal is output from the comparator, the first transistor is 'ON' so that current flows through the first resistor and the first transistor, so that the Is becomes larger than when the 'Logic High' signal is output. And the determined Is is an I ₄ value which is a current value of the fourth transistor by the current mirror circuit.

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